Variable resistor devices for "trimming" electrical circuits are quite well-known and have come into extensive use over the past years. in general, such devices, in contrast to potentiometers, are "set and forget" devices in which the resistance is varied until a preselected circuit condition is achieved. Every effort has been made to simplify these devices and to reduce the number of components in order to provide desired miniaturization compatible with the remaining components of a circuit.
Inevitably, these devices include a dielectric or insulating substrate having a resistive layer deposited thereon and two or more oppositely disposed terminals electrically connected to the resistive layer. In addition, the prior art devices have included a movable brush contact or brush member joined to another terminal before contacting the resistive layer and varying the resistance between respective terminals.
It will be obvious that there is a need for a device that the ultimate customer can simply set to a desired value, which in most instances requires a single setting for the life of the variable resistance device. A single in situ adjustment eliminates the need for a movable brush type contact. Eliminating all but the most essential elements greatly reduces cost and simplifies manufacture. It will also be apparent that there is always the possibility of the adjustable brush moving from the desired position under vibratory conditions, or by unintended contact during other circuit adjustments.
It is recognized that resistance trimming per se is well-known in the art. In fact, the assignee of the present application was also assigned U.S. Pat. No. 2,953,764 for a fixed resistor having a particular grid configuration, wherein certain sections of the grid could be removed to adjust resistive values. A somewhat similar device is illustrated in the Seay et al. U.S. Pat. No. 3,594,679 and U.S. Pat. Nos. 2,759,078 and 3,517,436 granted to Brown and Zandman et al., respectively.
It will be apparent, however, that although each of these patents disclose a means for removing a resistive film, such means either requires an elaborate scribing mechanism or other mechanism that is dependent upon "factory" trimming. No structures provide circuit trimming devices for field application by the ultimate customer, other than the relatively elaborate movable contact brush type constructions.
The Ghegan U.S. Pat. No. 1,618,003 and the Appleby U.S. Pat. No. 1,583,105 are illustrative of devices utilizing a means of applying or depositing a resistive track to a substrate by means of a graphite brush. These devices are relatively complicated and require that the brush remain on the track to complete the resistive circuitry between oppositely disposed terminals.